Eyeballing them by 'rolling up next to each other' is WHOLLY inadequate....



*YOU* dont measure them. YOU go to tire rack and - for each specific tire/brand/model- you look up the "specs".





Keep in mind that an "OD" isnt loaded (ie weight of the car on the tire) whereas the 'revs per mile' includes deflection when the tire is loaded.



Mismatch in the revs/mile is precisely what the differential 'sees'.. too much, it is damaged.